1. Field of the Invention
The present invention pertains to methods and apparatus used in oil exploration technology and more particularly to the methods and apparatus used to continuously measure the elastic properties of rocks located in subsurface formations.
2. Related Prior Art
Identification of subsurface rock properties is always important in oil well production since properties such as porosity and permeability can control oil recovery. Prior art has used correlations based upon acoustic travel times and bulk density to measure elastic rock properties such as bulk compressibility, shear modulus, Young's modulus, and Poisson's ratio. Rock properties are usually measured through the use of small samples that are taken from the well location and transported to a laboratory. However, laboratories cannot accurately measure rock properties in their original in situ conditions with the small rock samples that are usually transported to the laboratory.
Examples of patents that are indicative of prior art methods and apparatus for determining various rock properties are as follows.
U.S. Pat. No. 4,874,061, "Downhole Orbital Seismic Source" (Cole), relates to an apparatus for simultaneously generating elliptically polarized seismic shear waves and compression waves downhole for coupling energy through the fluid into the borehole wall. The apparatus includes an elongate frame for support in the borehole and the frame includes a drive means energizable to impart an orbital motion to at least a portion of the frame to generate an orbital shear wave.
U.S. Pat. No. 4,802,144, "Hydraulic Fracture Analysis Method" (Holzhausen et al.) relates to a method which uses a principle that the growth of a hydraulic fracture increases the period of free oscillations in the well connected to the fracture. Simultaneously, the decay rate of free oscillations decreases. The properties of force oscillations in a well also change during fracture growth. All of these effects result from the changing impedance of the hydraulic fracture that intersects the well. Hydraulic fracture impedance can be defined in terms of the hydraulic resistance and the hydraulic capacitance of a fracture. Fracture impedance can be determined directly by measuring the ratio of down hole pressure and flow oscillation. The fracture impedance can also be measured indirectly from well head impedance measurements using impedance transfer functions. Well head pressure measurements can also be used to evaluate fracture impedance by comparing them to pressure oscillations computed with hydraulic models that include fractures with different impedances. Since impedance is a function of fracture dimensions and the elasticity of the surrounding rock, impedance analysis can be used to evaluate the geometry of the fracture by analyzing the data which results from free and forced oscillations in the well, and looking for a match between the data and theoretical models of projected shapes of the fracture.
U.S. Pat. No. 2,244,484, "Method of and Means for Analyzing and Determining the Geologic Strata Below the Surface of the Earth" (Beers) relates to a method for seismically determining physical characteristics of subsurface formations which includes generating a sound in the vicinity of a formation. This sound is then transmitted into the formation by means of an electrical acoustic generator. The electrical quantities of which, measured at its terminals, depend upon the extent to which the power delivered to the generator is absorbed by the surrounding or adjacent formations. The variations in the measured electrical quantities indicate changes in the physical and lithologic characteristics of the various formations. These measurements are recorded by a graphical plot or log, or other means, at each successive position in the geologic section as a basis of correlation. This patent provides a method of seismically determining changes in physical characteristics of geologic strata which includes, setting up sound waves in the immediate vicinity of the formation or stratum, measuring the velocity of propagation of the sound through the formation and indicating the velocity at the surface. The characteristics of the formation may be readily determined by the time required for the waves to travel through the formation.
U.S. Pat. No. 4,802,144, (Holzhausen et al.) uses the measurement of hydraulic impedance to determine rock properties and U.S. Pat. No. 2,244,484, (Beers) measures downhole impedance (impedance log) to determine properties by determining propagation velocity. These two patents merely illustrate the state of the art in measuring rock properties in situ.